Method of storing spent nuclear fuel elements

ABSTRACT

For safety in storage and transport of nuclear fuel elements outside a nuclear reactor core, they are provided with a coating of a neutron-absorbing substance from a liquid phase, e.g. by immersion, spraying or pouring, utilizing a melt, a solution or immersion so that the possibility of critical mass attainment is eliminated or minimized.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of 06/679,461 filed Dec. 7, 1984 andrelates to the commonly assigned copending applications Ser. Nos.656,989 and 657,332, both filed Oct. 3, 1984. Ser. No. 06/657,332 hasbeen replaced by Ser. No. 06/902,006 filed Aug. 27, 1986 and is nowabandoned.

FIELD OF THE INVENTION

My present invention relates to a method of storing spent fuel elementsand, more particularly, burned out nuclear fuel elements which have beenremoved from a nuclear reactor core.

More specifically the invention deals with nuclear fuel elements,especially particles, balls or the like containing graphite or carboncoatings or bodies and which retain residual radioactive activity andmust be stored at least temporarily, e.g. for at least a certain degreeof decay of the activity.

BACKGROUND OF THE INVENTION

The term "nuclear fuel element" as used herein is intended to referprimarily to graphite or carbon-containing or coated bodies whichcontain nuclear fuel materials which have at least partly undergone afission reaction, although the invention may be applicable to othernuclear fuel bodies or elements as well. Generally the invention isintended to apply to spent fuel elements, i.e. fuel elements which havebeen removed from a nuclear reactor core because at least part offissionable material contained therein has undergone the fissionreactor.

It is known that nuclear fuel elements containing fissionable fuelmaterials undergo during their period within the reactor core only arelatively small amount of decay, fission or consumption, i.e. only asmall amount of the potentially fissionable material participates in thefission reaction.

During the fission reaction, fission products are formed which areconsidered neutron poisons, i.e. reduce the neutron generationcapability or reactivity of the fuel element. When the activity of thefuel element is diminished to a certain degree, it is common practice toremove the fuel elements and replace them by fresh fuel, the removedfuel elements being referred to as a spent fuel.

In general, only about 3 to 4% by weight of the potentially fissionablematerial in the fuel element can be considered to be consumed before thefuel element is deemed to be spent. The greater part of the fissionablefuel remains unreacted and thus unused so that it can be recovered bynuclear fuel processing procedures well known in the art.

Because of the high level of reactivity, spent fuel elements afterremoval from the reactor core are stored in radiation-shielding storagevessels to allow radioactivity to decay sufficiently to permitreprocessing. The storage must be effected under conditions such thatcriticality is not reached in the storage vessel, i.e. under conditionssuch that the self-sustaining chain reaction which results fromcriticality does not occur. This is achieved by an appropriate geometricarrangement of the fuel elements.

The storage vessels for this type of activity-reducing decay of spentfuel elements just removed from a nuclear reactor is generally effectedwithin the nuclear reactor installation itself.

After a satisfactory degree of decay, the spent fuel elements areamenable to reprocessing which is generally effected at a facility otherthan the nuclear reactor and may be transported and stored inappropriate transport receptacles. Naturally, during transport, caremust be taken to avoid any release of the radioactive material into theenvironment by accident or otherwise.

The initial decay storage of the spent nuclear fuel elements upon theirremoval from the reactor core generally utilized water tanks into whichthe nuclear fuel elements were dumped or lowered with appropriate careso that criticality would not be reached.

For both the initial decay period and for subsequent storage andtransport, however, there remains the need for a technique which willprevent or diminish the dangers represented by the activity of spentnuclear fuel elements, both with respect to reaching criticality andwith respect to release of radioactivity into the environment.

OBJECTS OF THE INVENTION

It is, therefore, the principal object of the present invention toprovide an improved method of storing spent nuclear fuel elements whichis capable of inhibiting the attainment of criticality and also reducesthe danger of release of radioactivity into the environment under normalconditions and in the case of accidents involving spent nuclear fuel.

Yet another object of the invention is to provide an improved method ofprotecting against the dangers of error in storage of nuclear fuelelements so as to preclude criticality even where a critical mass of thefuel elements has accidentally been accumulated in a geometry whichotherwise would cause a self-sustaining chain reaction.

It is also an object of this invention to provide a simple andeconomical way of reducing the hazards of spent nuclear fuel elements.

SUMMARY OF THE INVENTION

These objects are attained, in accordance with the invention, in amethod of storing spent nuclear fuel elements from a nuclear reactorcore in which the spent nuclear fuel elements are brought into contactat least over their surfaces, upon removal from the core, with aflowable neutron-absorbing substance capable of coating and depositingon the fuel elements and remaining adherent thereto for at least aperiod of storage which can include transport.

More particularly, the neutron-absorbing substance is in the form of asolution, emulsion or melt and is applied to the nuclear fuel elementsby immersing the fuel elements in this substance and/or by spraying orpouring this flowable substance onto the nuclear fuel elements.

Specifically, the spent fuel elements are immersed, sprayed or coated bypouring of the substance which is in the form of the solution, emulsionor melt and which remains adherent to at least the surfaces of the fuelelements which are uniformly coated with this substance. Theneutron-absorbing coating inhibits attainment of criticality no matterhow the fuel elements are assembled or stacked. The quantity of thecoating substance which is applied can be controlled easily byregulating the concentration of the substance in the solution oremuslion or melt and by the treatment time for the fuel elements, i.e.the immersion time, the duration of spraying.

An important advantage of the invention is that even in the case of anaccident in the storage vessel containing the solution, emulsion ormelt, no problem with respect to criticality results with the fuelelements because the adhesion of this substance to the surfaces of thefuel elements is such that the neutron absorption precludes attainmentof critical mass.

According to another feature of the invention, the fuel elements arerecovered, after coating with the substance in flowable form and driedin the case of a treatment with a solution or emulsion or subjected tohardening of the melt coating. The dried fuel elements can then beadditionally provided with a water-repellent coating. Alternatively, themelt can provide the water-repellent coating directly.

A preferred coating material is a solution of gadolinium acetylacetonate solution.

When the nuclear fuel elements are graphitic nuclear fuel elements, theyare immersed preferably in a gadolinium acetate solution in the bestmode embodiment of the invention. The gadolinium acetate solution can beapplied at its boiling point, thus promoting the penetration of thesolution into the graphite of the nuclear fuel elements and upondraining of the solution and cooling, the coated fuel particles arerecovered substantially in a dry state. They can then be coated with athin synthetic resin sheet or layer. Alternatively or in addition, aplastic foil or film can be applied in emulsion or melt form and cancontain the gadolinium acetyl acetonate.

In the cases described, an extremely adherent neutron-absorbent coatingor layer is applied to or formed on at least the surface of each nuclearfuel element to minimize neutron release beyond the fuel element.

The invention has been found to be especially applicable to graphitefuel elements, i.e. nuclear fuel balls or particles in which graphiteencases the uranium or thorium nuclear fuel. Such fuel elements areshown or described in the aforementioned copending applications whichapply to operations occuring within a nuclear reactor.

Upon immersion of these elements or spraying them with theneutron-absorbing substance or pouring the neutron-absorbing substanceover them, the neutron-absorbing substance does not merely adhere to thesurfaces, but penetrates into the pores of the graphite and fills thesepores after drying of the solution or emulsion or after solidificationof the melt. The fact that the neutron-absorbing substance is penetratedmore or less deeply into the nuclear fuel element not only improves thesafety during storage but also represents an improvement duringtransport and in the case of accident since even a traumatic disruptionof surface films will not eliminate the neutron-absorbing effect of theportion of the substance which has penetrated into the graphite.

The water-repellent coating which is applied serves to prevent washingoff of the coating and can be in the form of a lacquer such as shellac,a resin, a plastic, bitumen or even a nonwater-solublegadolinium-containing composition.

Indeed, when gadolinium acetyl acetonate constitutes the substance, itis found to be relatively refractory after drying to sprays of water orthe like which one might normally expect could wash off the substance.

Nuclear fuel elements of the type having discrete nuclear fuel unitsembedded in the graphite body can be in turn immersed in an aqueousgadolinium acetate solution. Here again penetration is promoted bymaintaining the gadolinium acetate solution or bath at the boiling pointand cooling the fuel elements after they are removed from the solution.

When synthetic resin foils are used to envelop the nuclear fuelelements, they can contain neutron-absorbing substances, the preferredmember of this class being europium oxides, gadolinium oxides, cadmiumoxides and mixtures or combinations thereof.

Depending upon the concentrations of these materials in the foils, thefoil thickness is chosen to preclude attainment of criticalityregardless of how the fuel elements are massed.

The foils are removed, e.g. by burning them off or dissolving thembefore the fuel elements are subjected to reprocessing.

I have found it to be advantageous, moreover, to provide such foils foreven fresh nuclear fuel elements to prevent criticality during storageor handling, the foils being removed before these fuel elements areintroduced into the nuclear reactor.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the presentinvention will bcome more readily apparent from the followingdescription, reference being made to the accompanying drawing in whichthe sole FIGURE is a flow diagram illustrating the invention.

SPECIFIC DESCRIPTION

As can be seen from the drawing, after removal of spent nuclear fuelelements at 10 from a nuclear reactor core 11, the spent fuel is sprayedwith a solution 22 or with an emulsion 13 or is immersed in either or iscoated with a melt 14 containing gadolinium or some other substance witha high neutron cross section to form an absorbent coating which is driedat 15 and 16 or solidified by cooling as shown at 17. The coated spentfuel particles are then further coated with a water-repellent film 18before they are subjected to storage at 19 for decay.

After the desired storage term and before reprocessing at 20, thecoating is removed at 21.

Fresh nuclear fuel is shown at 22 can also be coated with a foil, thistime containing the absorbing substance as shown at 23 and thus can bestored and handled at 24 without the danger of reaching criticality.Before the fresh fuel is introduced into the nuclear reactor, however,the coating is removed at 25.

SPECIFIC EXAMPLES Example 1

Spent metal sheathed nuclear fuel elements are removed from the nuclearreactor core into a tank containing cooling water in the form of asolution of gadolinium nitrate and gadolinium chloride although otherwater soluble gadolinium salts or water-soluble salts of otherneutron-absorbing elements can also be used. The solution concentrationscan be such that the neutron-absorbing substance is present in an amountbetween substantially 0.5% to the solubility limit of the water-solublesalt of the absorbing substance. Deposition of gadolinum on the metallicshell, effected galvanically by polarizing the metal shell negatively,i.e. as a cathode against an inert (e.g. stainless steel) anode andapplying a low-voltage direct current, e.g. about 24 volts. Dependingupon the activity of the nuclear fuel, coating was continued to build upthicknesses sufficient to reduce the neutron emission below any levelwhich can sustain criticality.

Example 2

Spent nuclear fuel elements are coated by spraying or dipping in asingle component or multicomponent lacquer, preferably an epoxy resinlacquer in which gadolinium oxide powder is dispersed in the lacquer.The lacquer is permitted to set or solidify.

Example 3

The fuel elements are coated by gadolinium acetyl acetonate by immersingthem in a melt of this substance or in an aqueous solution thereof or byspraying them with this solution or pouring this solution over them.Upon drying of the coating, the fuel elements are found to be waterrepellent and incapable of assembling into a critical mass.

Example 4

Spent graphitic fuel elements are introduced into a melt of gadoliniumacetyl acetonate and are stored in a solution of the melt at least untilgases begin to be driven off. The gases appear to be air which isexpelled by heat from the pores to promote penetration of the melt andthe solution so that these graphite pores are penetrated and at leastpartially filled with flowable material.

After drying, electron microscopy, autoradiography and neutronactivation analysis in the nuclear reactor shows a penetration of theneutron-absorbing substance into the deepest levels of the graphite fuelelements. The surface of the fuel elements were then immersed in hotsaturated aqueous gadolinium acetate solution and then cooled and dried.They are found to be filled with gadolinium acetate crystallites.

When the graphitic fuel elements are heated to a temperature aboveaboaut 750° C. after this treatment, the gadolinium acetate isdecomposed, leaving behind gadolinium oxide which is water insoluble andis immobile in the graphitic material even at high temperatures.

Gadolinium oxide thus becomes an integrated component of the spent fuelelement and resists loss therefrom even with major traumatic insults tothe fuel elements during accidents of transport or storage by theeffects of water and heat and even upon rupture of the fuel elements.

Example 5

Comparative tests with Example 4 were made with graphitic fuel elementstreated with aqueous gadolinium acetate at room temperature andatmospheric pressure. The amount of the neutron-absorbing substancetaken up by the fuel element increased with the residence time of thefuel element in the solution.

Example 6

Nuclear fuel elements are coated with a synthetic resin foil. The foilis a polyvinyl chloride and is applied in a melt which contains asneutron-absorbing substance europium oxide. The concentrations of thismaterial in the foil range from 0.5% to 25% by weight. Foil wraps ofpolyester, (Mylar) containing europium oxide as well as gadolinium andcalcium oxide also were used effectively. Before introduction of thefuel element into the reactor or reprocessing, the foil was removed aspreviously described.

I claim:
 1. A method of handling nuclear fuel elements which comprisesthe steps of:covering nuclear fuel elements with a thin synthetic resinlayer containing at least one neutron-absorbing substance; handling andstoring said nuclear fuel elements while they are covered with said thinlayer; thereafter removing the thin layer from said elements; andthereafter introducing said nuclear fuel elements into a nuclear reactorcore.